The majority of well reviewed Glasgow girls are pretty much 25+, Eva's 23 I think if that helps, though she's not back til October. Anyone on the younger end of 20 you're is going to require some research on your part. Take the link at the bottom of a girls page and fire it in the search bar up top and see if there's and reviews, otherwise you're going to have to TOFTT.
If it's your first time, I'd recommend staying with a British lass, and don't get so hung up on the age.